There are essentially two types of dyeing of keratin fibres: “permanent” dyeing and “semi-permanent” dyeing.
The first, also known as oxidation dyeing, uses “oxidation” dye precursors, which are colourless or weakly coloured compounds. Once mixed with oxidizing products, at the time of use, these precursors lead to coloured compounds and dyes via a process of oxidative condensation. In this case, the colorations obtained are generally very colourfast and strong.
The second, also known as direct dyeing, uses direct dyes, which are nonionic or ionic dyes and coloured compounds capable of producing a more or less pronounced change of the natural colour of the hair, resistant to shampoo-washing several times. These dyes may or may not be used in the presence of an oxidizing agent.
In contrast with oxidation dye precursors, a direct dye is a relatively voluminous molecule that does not penetrate easily into the core of the fibre. Consequently, even though considerable progress has been made in this field, the phenomenon of bleeding of the coloration during shampooing is still non-negligible, even if the dye(s) used is (are) chosen from cationic species.
Moreover, the use of certain cationic direct dyes may be reflected by a reduction in the working qualities of the shampoos used after coloration, especially as regards the duration of the lather.